In an electric power generation system that uses a combustion or gas turbine as a prime mover, a pressurized combustible fluid, such as propane, natural gas, kerosene or jet fuel, supplies energy to drive a plurality of turbine blades mounted on a turbine shaft. Air is compressed in a compressor stage and then directed to a combustion stage. Here the fuel mixes with the pressurized air and burns, with a resulting flow of high-pressure, high-velocity gas directed to the turbine blades, rotating the turbine to extract energy from the combustion process. The turbine shaft is axially connected to a generator shaft as a common shaft, and thus rotation of the turbine shaft imparts rotational energy to the generator shaft for producing electricity. Combustion turbine/generator combinations are available with various power output ratings and thus can be used to supply power for various applications, such as an industrial facility or an electrical power grid. Gas turbines are also used as aircraft engines.
It is known that gas turbines are not self-starting machines. For example, a separate starting apparatus may be connected to the common shaft of the combustion turbine generator to provide the starting rotational energy until the gas turbine reaches a desired rotational speed. The starter apparatus may then be disconnected and the combustion process supplies the energy to drive the turbine. The starting apparatus may include a starter motor; such as a DC (direct current) motor connectable to the common shaft through a transmission, and may include a clutch and/or torque converter in order to achieve a high starting torque. Typically, known electric starter apparatuses used for this type of application are operable just at a constant speed, hence the need of the transmission to develop the required high starting torques. It is also known that the starting apparatus may be in the form of a hydraulic starter, such as made up of a hydraulic motor responsive to a pressurized motive fluid as may be supplied under control of a hydraulic control system. It is noted that regardless of the specific implementation, such known starter arrangements are relatively expensive, consume valuable plant space and add incremental costs to the operation and maintenance of an electric power generation plant.
It is also known that because of its size and considerable weight, the rotatable turbine shaft of large combustion turbines may be susceptible to bowing if such shaft were to remain in one position for an extended period. One common technique used to overcome this problem that in the case of an electrically-driven starter may require use of an additional motor, such as a turning AC (alternating current) motor (generally referred to as a turning gear because the motor output is supplied to the turbine shaft via a gear box) for turning the shaft at a relative slow speed (e.g., 3 rpm) when the turbine/generator is not operating. This multi-motor arrangement may be employed since the starting torque required to rotate the turbine shaft from rest is much greater than the torque required to keep the shaft in rotation. For example, the electric motor starter may be sized for supplying the maximum torque requirement at starting and then the turning motor may be used to supply the torque required to keep the shaft just in relatively slow rotation. Needles to say, the foregoing arrangement exacerbates the issues noted above regarding costs and size of known starter arrangements.